Many different orthopedic casting materials have been developed for use in the immobilization of broken or otherwise injured body limbs. Some of the first casting materials developed for this purpose involve the use of plaster of Paris bandages consisting of a mesh fabric (e.g., cotton gauze) with plaster incorporated into the openings and onto the surface of the mesh fabric. Plaster of Paris casts, however, have a number of attendant disadvantages, including a low strength-to-weight ratio, resulting in a finished cast which is very heavy and bulky. Furthermore, plaster of Paris casts typically disintegrate in water, thus making it necessary to avoid bathing, showering, or other activities involving contact with water. In addition, plaster of Paris casts are not air permeable, and thus do not allow for the circulation of air beneath the cast which greatly facilitates the evaporation and removal of moisture trapped between cast and skin. This often leads to skin maceration, irritation, or infection. Such disadvantages, as well as others, stimulated research in the orthopedic casting art for casting materials having improved properties over plaster of Paris.
A significant advancement in the art was achieved when polyisocyanate prepolymers were found to be useful in formulating a resin for orthopedic casting materials, as disclosed, for example, in U.S. Pat. No. 4,502,479 (Garwood et al.) and U.S. Pat. No. 4,441,262 (Von Bonin et al.). U.S. Pat. No. 4,502,479 sets forth an orthopedic casting material comprising a knit fabric which is made from a high modulus fiber (e.g., fiberglass) impregnated with a polyisocyanate prepolymer resin such as polyurethane. Orthopedic casting materials made in accordance with U.S. Pat. No. 4,502,479 provide significant advancement over the plaster of Paris orthopedic casts, including a higher strength-to-weight ratio and greater air permeability. However, such orthopedic casting materials tend not to permit tactile manipulation or palpation of the fine bone structure beneath the cast to the extent possible when applying a plaster of Paris cast. In this regard, knit fiberglass materials are not as compressible as plaster, and tend to mask the fine structure of the bone as the cast is applied, e.g., the care provider may be limited in "feeling" the bone during reduction of the fracture. Although fiberglass fabrics are somewhat radiolucent they sometimes tend to mask the underlying bone structure to x-ray penetration. Oftentimes a fine mesh or a "shadow" can be seen on the x-ray image. This mesh, corresponding to the knitted fiberglass backing, obstructs the penetration of the x-rays and thereby obscures the fine detail of the underlying bone on the x-ray image.
Fiberglass backings have further disadvantages. Most, if not all, commercially available fiberglass casting bandages are comprised of filaments with diameters much larger than 3.5 microns (.mu.m). While 3.5 .mu.m fibers are considered by the scientific community to be non-respirable, there exists a sizable number of customers that have become concerned about the inhalation of fiberglass dust generated during cast removal. Moreover, orthopedic casting materials involving knit fabrics such as fiberglass are somewhat expensive, and may be cost prohibitive for some users.
An example of an orthopedic bandage using a polyester fabric which is not a knitted fabric is disclosed in U.S. Pat. No. 3,972,323 (Boricheski). However, the orthopedic bandage disclosed in U.S. Pat. No. 3,972,323 involves the use of plaster of Paris, and thus is subject to the disadvantages outlined hereinabove for plaster of Paris orthopedic casts, including an inferior strength-to-weight ratio and poor air permeability. A second example of an orthopedic bandage using a polyester fabric which is not a knitted fabric is disclosed in U.S. Pat. No. 4,841,958 (Ersfeld et al.). However, the polyester fabric backing disclosed in U.S. Pat. No. 4,841,958 causes the cast to have a somewhat lower strength and a lower rigidity than fiberglass casts. As such, these casting materials require more layers of casting tape to achieve a weight bearing orthopedic cast.
A cast material comprising a filled thermoplastic crystalline solid polyurethane is disclosed in U.S. Pat. No. 4,473,671 (Green). In use, the orthopedic cast material is warmed into a sufficiently high temperature to cause the polymer therein to become soft enough to deform. The orthopedic cast material is molded to conform to the surface shape of the effected portion of the body and then is cooled to room temperature. The filler of the casting material comprises a blend of 20% to 60% by weight of calcium metasilicate fibers and from 40% to 80% by weight silica particles. There are many inherent disadvantages with thermoplastic casting tapes including: (1) a separate heat source is required to cause the polymer to soften; (2) the "set" time is difficult to control due to variable initial and ambient temperatures; and (3) the cast material is prone to undesirable softening if warmed by the user.
From the foregoing, it will be appreciated that what is needed in the art is an orthopedic casting material which has both the advantages of plaster of Paris, e.g., good moldability and palpability of the fine bone structure, and the advantages of non-plaster of Paris materials, e.g., good strength-to-weight ratio and good air permeability. In this regard it would be a significant advancement in the art to provide such a combination of advantages without actually using plaster of Paris, thereby avoiding the inherent disadvantages of plaster of Paris outlined herein. It would be a further advancement in the art to provide such non-plaster of Paris orthopedic casting materials which have as good or better properties than the orthopedic casting materials of the prior art, and which can be made to be significantly less expensive, and therefore less cost prohibitive, than prior art orthopedic casting materials employing knitted fabrics such as fiberglass knits. Such orthopedic casting materials and methods for preparing the same are disclosed and claimed herein.